The Institute of Electrical and Electronics Engineers (IEEE) 802.11 is a set of Media Access Control (MAC) and PHYsical layer (PHY) specifications for implementing Wireless Local Area Network (WLAN) computer communication in the 2.4, 3.6, 5, and 60 GHz frequency bands. They are created and maintained by the IEEE Local Area Network (LAN)/Metropolitan Area Network (MAN) Standards Committee (IEEE 802), The base version of the standard was released in 1997, and has had subsequent amendments. The standard and amendments provide a local area wireless computer networking technology that allows electronic devices, e.g. an AP and a station (STA), to connect wirelessly to a network. A WLAN is sometimes referred to as a WiFi network.
In e.g. IEEE 802.11, the most commonly used channel access is distributed among the different stations (STAs). This means that a STA before a transmission needs to sense whether the channel is idle or busy, and only if the channel is found to be idle, a transmission can take place. This is a very simple approach, but still it does in many situations perform very well. This protocol is commonly known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). However, in order to work properly, the different STAs need to hear each other, e.g. they need to be located within radio coverage with each other. There are situations when e.g. two STAs connected to the same Access Point (AP) do not hear each other, as illustrated in FIG. 1. In this figure, a first STA, STA1, and a second STA, STA2, are both connected to the AP, but due to that the distance between the two STAs is large the second STA, STA2, cannot detect when the first STA STA1 is transmitting to the AR In the figure the circles are used to illustrate the respective coverage area, e.g. the respective radio coverage area, of the first STA, STA1, and the AP, respectively. This means that even if the first STA, STA1, is transmitting to the AP, the second STA, STA2, will not be able to detect this and therefore the second STA, STA2, may determine that the channel is idle and may therefore start its own transmission to the AP, which may cause a collision at the AP.
A Request-To-Send (RTS)/Clear-To-Send (CTS) procedure is a handshaking procedure. Before sending a packet, the transmitter, e.g. the AP, sends an RTS and waits for a OTS from the receiver, e.g. the STA. The reception of a CTS indicates that the receiver was able to receive the RTS, and that it is ready to receive data packet(-s) from the transmitter, i.e. the radio channel is clear in its area.
Further, the use of RTS and/or CTS is a means to prevent issues with Hidden Nodes (HN) and is described in e.g. IEEE 802.11, FIG. 1 schematically illustrates a hidden node problem according to the prior art, Referring to FIG. 1, when a first STA, STA1, senses, e.g. detects, the channel as being idle, it sends an RTS packet to an AP, and if the AP receives this it sends a CTS packet. A second STA, STA 2, cannot hear the RTS packet, as the first STA, STA1, is a HN for the second STA, STA2, but it can receive the CTS packet sent by the AP. The RTS and CTS packets comprise information about that the channel will be occupied for a certain amount of time, and thus the channel should not be accessed even if sensed idle. Thus, once the second STA, STA2, receives the CTS, it will effectively obtain information about that the first STA, STA1, will transmit to the AP and will during the time indicated in the CTS packet defer from transmitting.
Two things are worth pointing out. The first thing is that the use of RTS and/or CTS does not completely avoid the HN problem as there is a probability that the second STA, STA2, begins a transmission while the first STA, STA1, is sending the RTS packet. However, the RTS packet is typically much shorter than the actual data packet, so that the probability of a collision is significantly reduced. The second thing is that the use of RTS and/or CTS effectively means increased transmission overhead, and consequently reduced throughput.
Now, FIG. 1 and the discussion above illustrated the HN problem within one cell, often referred to as a Basic Service Set (BSS) in IEEE 802.11, and where the RTS/CTS was used to protect an Up-Link (UL) transmission. However, the HN problem may also occur in the Down-Link (DL), when e.g. STAs connected to another AP do not hear a DL transmission from the AP. This is illustrated in FIG. 2, where the first AP, AP1, cannot hear the transmissions that may occur from the second STA, STA2, to a second AP, AP2. In this case a first AP, AP1, may send an RTS to the first STA, STA1, which in turn may respond with a CTS. The second STA, STA2 will hear this CTS and will not initiate a transmission to the second AP, AP2, even if the channel is sensed being idle.
With the introduction of Multi-User (MU) transmission in the DL, the use of RTS and/or CTS may cause a significant overhead. Specifically, suppose that the AP is about to protect a DL transmission to four STAs STA1-STA4, as illustrated in FIG. 3. The AP then sends an RTS to the STAs STA1-STA4, and the STAs STA1-STA4 send the CTS back to the AP. This can be done in several ways. It is here assumed that only one RTS packet is sent, which addresses all STAs. However, how to send the CTS packets leaves more options. One may for instance be to let the STAs send the CTS one at time which will give detailed information about the channel conditions at each one of the STAs, but at the cost of a rather lengthy CTS transmission as there in this example will be four transmissions. This implies that such a solution does not scale well when the number of STAs grows.
US 2014/0341135 A1 to Bhushan et al. describes the signaling of RTS signals and CTS signals in an unlicensed spectrum. An eNB sends an RTS signal and the UEs named or served by the transmitting eNB send a common CTS signal a short time after the receipt of the RTS signal. The common CTS allows the UEs to grab the channel as quickly as possible. Further, the UEs identified by the transmitting eNB may send individual CTS signals staggered in time. The staggering may depend on the order in which the UEs are ready to receive data. Each of the individual CTS signals may comprise a MAC ID of the eNB transmitting the RTS signal and a MAC ID of the UE transmitting the individual CTS signal. A drawback with the staggering in time of the individual CTS signals is that the time period during which a CTS signal is valid is limited. For example, if the period of time between a first point in time when the first UE transmitted its individual CTS and a second point in time when the actual transmission from the eNB is to take place is too long, for example, due to the fact that other individual CTS signals staggered in time are to be received before the transmission can take place, the first UE may no longer experience a channel that is free. Thus, a transmission to that first UE will not be received by the first UE and will only increase the transmission overhead and thereby reduce the throughput. Further, the described staggered CTS signaling results in lengthy CTS transmissions, which as mentioned above, is not suitable when the number of UEs increase in the communications network.
In order to circumvent the potentially very high overhead with sequential CTSs, in IEEE 802.11-15/0867r1 (Po-Kai Huang, MU-RTS/CTS for DL MU) a method for simultaneous CTS transmission from different STAs to the AP has been proposed. As before, the RTS/CTS mechanism is used to protect against hidden nodes. However, now a more efficient RTSiCTS mechanism for protecting DL MU transmissions from the AP is proposed.
The mechanism is as follows. First, the AP transmits a MU-RTS frame which contains (among other fields) the addresses of the STAs that it wants to transmit to in the subsequent DL MU transmission. The STAs decode the MU-RTS, and only the ones that have their address listed in the MU-RTS frame respond with a CTS frame. In the new proposal, the CTS transmissions are transmitted simultaneously by all the STAs. Moreover, their content s exactly the same. Hence, as a result, this results in multiples copies of the same packet being received at the AP, from different paths and different delays. This is a typical multipath fading scenario that the AP knows how to handle. As a result, the transmission time of the CTS is minimized, since all the STAs transmit at the same time (in contrast to TDD in previous standards), and ensuring that the content is the same makes it easy to receive.
However, although the transmission time is minimized by simultaneously transmitting the same CTS packet, it has the drawback that the AP cannot detect which ones of the addressed STAs actually sent a CTS packet. The only thing the AP will be able to detect s whether at least one of the STAs sent a CTS packet. Therefore the AP may transmit its transmission to STAs not able to receive it. This can result in unnecessary transmissions causing increased transmission overhead and reduced throughput.